521666561 has 4 divisors (see below), whose sum is σ = 561794772. Its totient is φ = 481538352.

The previous prime is 521666557. The next prime is 521666581. The reversal of 521666561 is 165666125.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 76667536 + 444999025 = 8756^2 + 21095^2 .

It is a cyclic number.

It is not a de Polignac number, because 521666561 - 2² = 521666557 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (521666501) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 20064086 + ... + 20064111.

It is an arithmetic number, because the mean of its divisors is an integer number (140448693).

It is a Proth number, since it is equal to 995 ⋅ 2¹⁹ + 1 and 995 < 2¹⁹.

Almost surely, 2^521666561 is an apocalyptic number.

It is an amenable number.

521666561 is a deficient number, since it is larger than the sum of its proper divisors (40128211).

521666561 is a wasteful number, since it uses less digits than its factorization.

521666561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 40128210.

The product of its digits is 64800, while the sum is 38.

The square root of 521666561 is about 22840.0210376436. The cubic root of 521666561 is about 805.0033105583.

The spelling of 521666561 in words is "five hundred twenty-one million, six hundred sixty-six thousand, five hundred sixty-one".